The present invention relates to a mass manufacturing method for a display element for a timepiece and a display element manufactured in accordance with such method.
The term,  less than  less than display element less than  less than , must be taken here in its broadest sense. It may concern a dial, a date disc or moving figurines showing, for example, phases of the moon, provided however that these watch-parts have an undulating and thus not flat profile. In order to manufacture such parts on a large scale, they thus not only have to be cut from a strip, but have to be shaped, prior to cutting, in accordance with a profile suitable for their functions.
Manufacturing such watch-part from a metal strip is known. If one wishes to manufacture a date disc, for example, one begins by stamping the strip to give it the three levels required by the disc profile. The inner toothing is then cut, then the disc is detached from the strip by peripheral cutting. The stamped discs are then nickeled and washed, then transferred loose in bulk to a station where paint is applied to one of their surfaces. The paint is dried, then the figures of the date are transferred.
The method which has just been described has several drawbacks. it will be mentioned first of all that when they are transported loose in bulk to the painting station, the discs can become warped or become caught in each other so that the teeth can be damaged. This can lead to a significant rejection rate following a time-consuming check. It will also be noted that the nickeling and washing steps which precede transportation are long and expensive. Then, during the painting step, which is also long and expensive, it is very difficult, given the dimensions of the discs, to prevent the paint being deposited on the toothing, such a deposit being capable of braking the disc after the mounting thereof and thus causing it to operate unsatisfactorily. It will further be noted that prior to transferring the figures, it is necessary to orient the disc so that the inscriptions are transferred in a very precise manner with respect to the teeth of the disc, failing which said inscriptions may not be centred with respect to the window through which they are read. It will be understood that this step is also time-consuming. Finally, since the disc is made of metal, it will be understood that it is necessary to oil it at the location where it rubs against the movement plate, which constitutes an additional step which is also time-consuming.
It will be observed that many of the drawbacks cited hereinabove would disappear if the metal disc were replaced with a disc made of plastic material and if loose bulk transportation of the discs could be avoided.
Date discs made of plastic material have already been proposed. This is the case for example of Swiss Patent No. 554 554 which discloses a manufacturing method wherein the date indicator is made of a flat ring made of plastic material to which a metal ring having an inner toothing is ultrasound welded. This embodiment allows the aforementioned problem of paint overflow to be prevented, since the flat plastic ring does not need to be painted. However, other drawbacks remain in addition to the newly created drawback of resorting to a part made of two components which is inevitably expensive to manufacture.
Swiss Patent No. 544 332 also discloses a method for manufacturing a date indicator which is entirely made of plastic material. The plastic injection moulding technique is implemented here. However, a reading of this Patent shows that enormous precautions must be taken to inject a disc whose thickness is of the order of 0.3 mm, both as regards the selection of the material and the construction of the mould. This thus leads to a part which is expensive and even impossible to manufacture if the thickness of the disc should reach 0.15 mm, as can be the case for the disc made by means of the method of the present invention.
Thus in order to avoid a large number of the drawbacks listed above the present invention relies advantageously on a method which, although known in itself, has never been used to make a display element intended for a timepiece. The mass manufacturing method for a display element, in particular for a timepiece, is characterised in this new invention in that it includes the following series of steps:
a) a sheet of plastic material in a tape is provided, the width of the tape having sufficient size to manufacture at least one display element;
b) the sheet is deformed by thermoforming to give it a profile corresponding to that of the finished display element; and
c) the display element is separated from the tape by cutting, this cutting corresponding to the outer contour of said display element.
In an even more advantageous manner, the method set out above is completed by introducing after step b) and before step c) a step d) which consists in making at least one cut-out portion in the inner portion of the display element, and a step e) which consists in transforming indications on said display element. Finally, in order to finish off, and for reasons which will appear hereinafter, one could, after step c), stack the display elements in a magazine.
The present invention also concerns a display element made in accordance with the method set out above, the description which follows applying above all to the description of a date disc.